Well, It would be good to know if it is reference design(being a BFG 6800GS it probably is though).
The following two are reference design. Having a reference design board is not mandatory unless I am mistaken but it greatly increases the possibility of the card being usable as chances are there is nothing extremely out of the ordinary on it.(something that would require a custom ROM)Picture of two 6800 cards here (picture taken by "OriginalMacnut", original thread here. )
Got some more reference design boards here as well.
Another thing is that it cannot be one of the later ones with a HSI bridge chip integrated into it because if I remember correctly those used NV42 GPUs instead of the normal NV40. If my information is correct the ones with a bridge chip needed that bridge because they were actually PCIe hardware on a board with an AGP connector using the bridge to go from PCIe to AGP.

Again, unless I am mistaken a 6800GS AGP is just a 6800GT AGP but the 6800GS AGP has four pixel pipes and one vertex pipe disabled.
Someone by the name of hamstrings on another forum has previously said the following.

Quote:

Originally Posted by hamstrings

Have a look at the 6800 GT AGP reference design pictures with Google. If the cooler matches the original design, then you might have a NV40 core for the 6800 GS AGP. There would be no room for the PCIe-AGP bridge.

And on the topic of pipelines:

Quote:

Originally Posted by hamstrings

They can be tested using RivaTuner and then reenabled by editing the Mac ROM. They are not always usable though: my 6800 GS AGP has a defective quad that causes severe artifact on screen.

RivaTuner *should* also be able to report GPU version as well.

So, in the end it seems NV40 GPU and no PCIe-AGP bridge. At the moment there is no 128MB 6800 ROM available (only 256) but editing ROMs of the 6600 and 6800 are somewhat documented so it should be possible without too much bloodloss.